Domitius Ahenobarbus, vowed to build a temple for the god of the sea after a naval victory, perhaps the one won off Samos in 129 or 128 BC against Aristonicus who had attempted to oppose the donation of Pergamon to Rome by the will of King Attalos III.

[3][4] The reliefs, from the base of the cult statue, which originally consisted only of mythological scenes were brought to completion after Domitius Ahenobarbus' censorship in 115 BC with a fourth panel.

[m 2] The reliefs are mentioned in 1629 and 1631 after having been uncovered during the works undertaken by the Roman Santacroce family between 1598 and 1641, which included notably the construction and expansion of a palace near the Tiber by the architect Peparelli.

[6] Despite their common name, the reliefs probably did not originally form part of an altar, but a large rectangular base (5.6 m long, 1.75 m wide, and 0.8 m high),[7] intended to support the cult statues.

[3] These included works attributed to the Greek sculptor Scopas,[n 4] like the important statuary group featuring Neptune, Thetis, Achilles, the Nereids, Tritons and Phorcys with sea monsters and other fantastic creatures.

The large panel conserved in the Louvre is typical of Roman civic art and depicts a scene which one would only find at Rome in this period: the census.

[6] The mythological frieze seems to have been executed earlier, covering the three visible sides of the base, which would originally have been connected to the back wall of the cella.

[3] The relief, which is one of the first examples of the continuous narrative style,[m 3] is read from left to right and can be divided into three scenes: the recording of the Roman citizens in the register of the censor, the purification of the army before an altar dedicated to Mars and the levy of the soldiers.

[m 4] The identity of this man is not absolutely certain, his identification as the iurator is challenged by a detail: he appears to be wearing calcei, special shoes reserved for individuals of Senatorial rank.

By this gesture (the manumissio), the censor accepts his declaration and renders his judgement (the discriptio), which concluded the process of assigning the individual to a class.

At right, three sacrificial victims are led up, the bull, the ram and the pig (the suovetaurilia) which were to be sacrificed in honour of the god in order to secure good fortune for the departure of the troops.

[3] The panels bearing the mythological scene, a marine thiasos of late Hellenistic style, are made of a different marble derived from Asia Minor.